The requirement of a high starting voltage is a well-known characteristic of discharge lamps. Some types of discharge lamps, such as high-intensity discharge (HID) lamps, require starting voltages on the order of several thousand volts.
The prior art teaches a number of starting circuits for HID lamps. Many prior art starting circuits employ a pulse coil to generate a narrow high voltage pulse for igniting the lamp. Typically, the pulse coil is located in series with the lamp and must therefore be capable of handling the current that flows through the lamp after the lamp ignites. Because the lamp operating current is typically quite large (e.g., 1 ampere or more in the case of a 100 watt M98 type lamp), the pulse coil must be wound with fairly large diameter wire in order to keep resistive power losses within a manageable limit and thereby preserve the energy efficiency of the associated ballast. Consequently, the pulse coil may have considerable physical size and monetary cost.
“Pulse” type starting circuits typically require breakdown devices such as sidacs. Such devices add significant cost and/or complexity to the starting circuit, and may significantly detract from the overall reliability of the associated ballast. Moreover, “pulse” type starting circuits are usually ill-suited for use with remote installations in which the length of the wires between the lamp and ballast is more than a few feet. Due to the inherent capacitance of the wiring, the high frequency starting pulse may be significantly attenuated. Consequently, the lamp may not receive sufficient voltage to ignite.
The prior art also includes a number of alternative approaches for igniting an HID lamp. Examples of such circuits are disclosed in U.S. Pat. No. 6,008,591 (Huber et al.) and U.S. Pat. No. 6,362,576 B1 (Huber et al.), both of which disclose ignitor circuits which appear to represent a considerable advance over the prior art.
FIG. 1 describes an ignitor circuit 100 like that which is disclosed in U.S. Pat. No. 6,008,591. During operation of ignitor circuit 100, CIGN charges up from an alternating current (AC) voltage, VAC, (provided between input terminals 102,104) via D1 and RIGN. When the ignition switch [i.e., transistor QIGN, which is realized by an insulated-gate bipolar transistor (IGBT)] is turned on by an appropriate control signal VG, the stored energy in CIGN is discharged through the primary winding 122 of transformer 120, thereby inducing a voltage across primary winding 122. The induced voltage across primary winding 122 correspondingly induces a voltage across secondary winding 124, which provides a high voltage, VIGN, (between output terminals 116,118) for igniting the lamp. The desired amplitude for the ignition voltage, VIGN, is obtained by selecting an appropriate turns ratio for the primary and secondary windings 122,124 of transformer 120.
A drawback of ignitor circuit 100 is that it does not provide a positive current path if/when transistor QIGN is turned off before the current through transistor QIGN (i.e., the current that flows from the collector to the emitter when QIGN is turned on) falls to zero. Consequently, ignitor circuit 100 requires that the control signal VG be removed (i.e., set at a value—e.g., zero volts—that is insufficient to activate transistor QIGN) when anti-parallel diode DA is conducting, which tends to increase the complexity and cost of providing a suitable control signal, VG.
Another drawback of ignitor circuit 100 is that the peak current that flows through transistor QIGN is relatively high (e.g., 30 to 40 amperes). During operation of ignitor circuit 100, the leakage inductance of primary winding 122 acts to limit the amplitude of the discharge current (from the stored energy in CIGN) that flows when transistor QIGN is on. Unfortunately, in ignitor circuit 100, the leakage inductance of primary winding 122 is quite low in value, which has the effect of allowing the discharge current to have a relatively high peak amplitude (e.g., 30 to 40 amperes). Consequently, in implementing ignitor circuit 100, transistor QIGN must be realized by an IGBT having a high peak current rating; that, of course, tends to increase the material cost of implementing ignitor circuit 100.
FIG. 2 describes an ignitor circuit 200 that is a modified version of ignitor circuit 100 (described in FIG. 1). Ignitor circuit 200 includes a pair of DC input terminals 202,204 for receiving a substantially direct current (DC) rail voltage, VRAIL, a pair of AC input terminals 206,208 for receiving an alternating current (AC) voltage, VAC, and a pair of output terminals 216,218 across which is provided an ignition voltage, VIGN. In comparison with ignitor circuit 100, ignitor circuit 200 includes one additional component—an inductor LIGN. During operation of ignitor circuit 200, inductor LIGN serves to limit the rate of rise of the current that flows through ignition switch QIGN; clamping diode DCLAMP provides a current path when QIGN is turned off.
During operation, ignitor circuit 200 provides an ignition voltage, VIGN, for which the amplitude of the positive peak is generally higher than the amplitude of the negative peak (which is much less than 3 kilovolts); it has been observed, however, that the amplitude of the positive peak is sometimes significantly reduced and actually lower than the amplitude of the negative peak, such that the amplitudes of the positive peak and the negative peak are both less than 3 kilovolts. It is believed that this reduction in the amplitude of the positive peak of VIGN is attributable to the influence of parasitic components, such as the leakage inductance and the coupling capacitance of transformer 220 (of which the leakage inductance is prominent).
In practice, in order to provide a sufficiently high peak value for VIGN, ignitor circuit 200 requires a relatively high turns ratio between the secondary and primary windings 224,222 of transformer 220; for example, it has been contemplated that, for a DC rail voltage (VRAIL) of about 460 volts, ignitor circuit 200 requires 96 turns on secondary winding 224 and 13 turns on primary winding 222 (which gives a secondary-to-primary turns ratio of about 7.4). Unfortunately, a high turns ratio is also accompanied by a high leakage inductance for transformer 220, which tends to aggravate the previously noted negative impact upon the relative amplitudes of the positive and negative peaks of VIGN.
What is needed therefore, is an HID lamp ignitor circuit that provides an ignition voltage having a reliable magnitude that is less affected by parasitic components. A need also exists for an ignitor circuit that is capable of being realized in a more cost-effective manner than comparable prior art approaches. Such an ignitor circuit would represent a considerable advance over the prior art.